Determination of metals in animal tissues by potentiometric stripping analysis without chemical destruction of organic matter

Ekrmhlmka

Pergamon

Acta, Vol 39 No 3. PP. 317-325.1994

C.~pynght01!394 EkweSacnaUd PtmtcdmGmtBntam.AUnghtsracmd aI&4686/w

$600 + a00

DETERMINATION OF METALS IN ANIMAL TISSUES BY POTENTIOMETRIC STRIPPING ANALYSIS WITHOUT CHEMICAL DESTRUCTION OF ORGANIC MATTER CH

LABAR

and L LAMBERTS

Laboratolre d’Electrochlmle et de Chlmle Analytlque, Facultes Umversitames Notre-Dame-de-la-Palx, 61, rue de Bruxelles, B 5000 Namur, Belgmm (Received 19 July 1993, IIIrewed form 9 September 1993)

Abstract-Metal (Cu, Pb, Cd) ions contents of various ammal tissues have been determmed by potentlometnc stnppmg analysis (PSA) from lyophdlzed samples The orgamc matter destruction, which ISthe classical prehmmary step of such determmatlons, IS circumvented by the use of sodmm dodecylsulfate (DOC) and sodmm desoxychohc acrd (SDS) solutions, m a reflux process and wth the ad of somcatlon The effiaency of the digestion IStested by comparison with total mmerahsatton with the low temperature ashmg (LTA) method The hgestmg medium IS adapted to PSA measurements by the ad of a mixed (aqueous-orgamc) electrolyte Operating PSA parameters have been studied m the final medium to ophmaze analytical condltlons m the goal of adapting the entire process for routine analysis m environmental

control studies The inherent advantages of a no-current electrochemical method are demonstrated The process uses classical mstrumentatlon and can be conducted, from the lyophlhzed sample, m approxlmately 3-4 h of operation The relative standard devlatton obtained by standard additions method IS *5% for Cd and Pb, and k3% for copper The study IS conducted with various samples of hafer and fish livers obtained from the commercial dlstnbutlon clrcmt and by fishing sampling. Key words potentlometnc stnppmg analysis, ammal tissues, heavy metal determmatlon, digesting proCdUE

INTRODUCTION

Heavy metal determmatlon m animal tissues 1s of great importance for pollution studies The wet or dry decomposltlon of the orgamc matter 1s the almost universal prehmmary step m such determmations[ 1) Loss nsks and contammatlon problems are unfortunately the unavoidable companions of the multlphclty of operations and of the number of chenucal agents added between organic matter destruction and analytical SlgnBl measurement[2,3] When electrochemical detection 1s used, prehmlnary steps interfere with the analytical signals by erther shfitmg the half wave potentials or by maskmg one of the metals to be determined, as IS the case m polarography and anodlc stnppmg voltammetry[4] A self optmusmg method clrcumventmg such dl& cultles and which can be performed with a mmlmum of prehmmary steps, 1s thus of great interest The goal of this paper 1s to demonstrate the unneoessarmess of total organic matter destruction and the posslblhty of the direct determmatlon of the heavy metal from the medium, obtained through another prehmmary step, when potentlometrlc stnpping analysis (PSA) is used as the electrochemical method of analysis, m the field of routme analysis of animal tissues THEORY The PSA technique 1s based on potentlostatlc reduction and amalgamation of the metal ions,

M(n+) followed by a potentlometnc measurement The analysis thus cons& of two steps an electrolysls step and a stnppmg step Durmg the electrolysis, the metals are accumulated m a mercury film on the surface of a glassy carbon workmg electrode to which a reducing potential, Ed has been apphed Hg(II+) + 2 e- - Hg(l) M(n+) + n e- - M(Hg) During the stnppmg step, the apphed electrolysis potential E, IS removed and no current passes through the electrodes, a chemical oxldlzmg agent (here, excess mercunc ions) stnps the amalgamated metals off the electrode Hg(I) + Hg(I1+) - Hg; + M(Hg) + n/2 Hg(II+) - M(n+) + (n/2 + 1) Hg(1) and the metal ions diffuse back to the solution Measurement of the electrode potential as a function of time provides quantltatlve as well as qualitative mformatlon about the metals present In actual expenments, the only chemical reagent needed to perform PSA is mercuric ions, Hg(II+), which serve as a source of ions m the mercury film formation (electrolysis step) and as the chemical 0x1dlzmg agent durmg the redlssolutlon (stnppmg) step The PSA theory has been extensively described elsewhere[4-61 and only the final relatlonshlps are needed here The analytical basis relation 1s t, = kC*M(n+)t,

317

(1)

318

CH LABARand L LAMBERTS

where C*M(n+) 1s the bulk metal ion concentration to be measured, t, IS the electrolysis time and t, IS the strlppmg time, which 1s the analytically measured signal When the drffuslon layer thickness 1s allowed to reach Its maximum value dunng only the stnppmg step, by carrying out this stage m a quiescent solutton, the velocity of the re-oxldatlon process IS lowered and the strlppmg time 1s evidently enhanced, for the same value oft, this 1s the theoretical background of so-called “stationary electrode PSA” methods[7, S] The quahtatlve metal determmatlon 1s based on the Nernst equation for the amalgamated metals E = E” + RT/nF ln([M(n+)l/CM(Hg)l),

(2)

where E” IS the redox potential, n IS the number of electrons mvolved m the strlppmg step, CM@+)] 1s the metal ion concentration m the dlffuslon layer at the electrode surface, [M(Hg)], the concentration m the mercury film, F IS the Faraday constant and T IS the absolute temperature Accordmg to the equation (2), the order m which the amalgamated metals are oxldlzed IS a function of the E” value of the mdlvldual metals, furthermore, the method 1s self optlnnzmg because one metal can only be stripped after the preceding metals m the E” classification have been already be[9] Radial dlffuslon can also be substituted for linear dlffuslon by the use of the mlcroelectrode[4,10], a rotatmg disk carbon electrode can be used as a working electrode[ll, 121 the technique 1s thus very apt to use new sensors Furthermore, m PSA, the potential change of the workmg electrode 1s measured as a function of time and the determmatlon 1s not based on current measurements this ehmmates measurement errors caused by the hqmd resistance between the electrodes (the Ill-drop) and the potential drop over the electrical double layer around the working electrode The no-current measurement reduces the background signal and provides the posslbthty of analysmg samples with a high content of organic redox compounds without these mterfermg with the metal determmatlon Sample preparation IS generally very simple and deaeratlon 1s often unnecessary smce dissolved oxygen can be used as an oxidant (for high metal contents) The large dynamic range (from 1OOppm to sub-ppb levels) enables measurements to be performed directly on the sample A recent review paper by Manmno and Wang[ 131 demonstrates the analytical posslblhtles of PSA m the analysis of a wide variety of samples and presents this technique as an mterestmg alternative when other electrochemical methods (such as anodlc strlppmg or square wave voltammetry or dlfferentlal pulse polarography) fall Recommended methods of analysis of metallic Impunties in orgamc materials involve a total wet or dry decomposltlon of the sample[l], such methods include numerous chemical steps and operations, and are thus time consummg and, m some cases, hazardous[l4], loss and contammatlon risks are frequently observedC2, 3, 141 Here, we investigate a softer process mvolvmg only the redlssolutlon of the metal ions bonded m the orgamc material, wlthout ashmg or decomposltlon Such processes have already been studied by the use of specific agents

(Lumaton, ammonium quaternary amine, etc) for hologcal materials,, when, ge ac polarography IS the final analysis step[15, 163 We study here the use of more common and safer agents, essentially the sodmm salts of dodecylsulfate (DOC) and desoxychohc (SDS) acids, mixed m aqueous solution, completed by the posslblhtles of lyophlhsatlon and sonochemlstry[ 171 It can be demonstrated that such a procedure 1s effective without a supplementary step and leads directly to a medium compatible with routme PSA measurements, wlthm a largely reduced operational time EXPERIMENTAL Apparatus and reagents

The basic unit consists of the Radiometer ISS 820 Ion Scanning System[7, 81, It acts as a potentlostat durmg electrodeposltlon and, after automatic dnconnectlon at the end of this step, as a chronopotentlometrlc device during stnppmg, coupled Hrlth the associated recorder (REA 80) for the vsuallsatlon of the E-t curve A three-electrode system 1s used Radiometer P 1312 (Pt) 1s the counter electrode, Radiometer F 3500 1s the glassy carbon electrode which, before measurements, 1s modified as a mercury thin film (MTF) working electrode[7], and Radiometer F 4040 1s the see reference electrode Insulation of the workmg and counter electrodes 1s enhanced by appropriate shielding The five entry electrochemical cell (Metrohm 6 5703 150) 1s thermostated at 25°C Nitrogen 1s used to deareate the solution The low temperature asher 1s the TLA 504 from LFE Process Control Dlvlslon, with four ozomsatlon rooms[18] Lyophlhsatlon IS made with the Edwards Freeze Dryer Modulyo type 6110 Homogenization and somcatlon are performed with routme commercial instrumentation All chemicals are of analytical reagent grade and redlstdled water 1s used The mam chemicals used are DOC (Aldnch 862010), SDS (Aldrich 107301), used from 20% weight stock solutions prepared twice a month, mercunc nitrate monohydrate (Aldrich 23413) or m the form of the correspondmg volumetric standard (Aldnch 319058) and a mixture of Cu, Cd and Pb atomic absorption standard solutions m HNO, 0 05 M The reference solution used to calibrate PSA instrumental settmgs 1s the test solution developed by RadIometer (Radiometer S 2202 test solution) All PSA measurements are made Hrlth a background correction recorded on the real sample studied In the followmg paragraphs, N 1s the symbol of the number of independent measurements performed to have the numerical mean appearing m Figures and Tables, N’ IS the symbol of the number of mdependent samplmgs of a gven sample RESULTS AND DISCUSSION Dzgestlon procedure and adaptation to PSA method General procedure The procedure follows five lyophPsation, homogenization, steps main dlgestlon, adaption to and PSA measurements

PSA of metals m ammal tmws

319 d

>

4‘

Fig 1 PSA signals of a hder hver tissue at the end of the LTA process m reference perchlonc acid medium Electrodepoatlon potential, E, = - 1 lOV/sce, electrodeposltlon tune, ta = 1 mm, mode stationary electrode PSA, electrolytic medmm HCIO, 0 52 M, Hg ++64x 10m5M CdandPbanalysls (a) background curve of the sample, (b) stnppmg curves of the sample, N = 3 Cu anaIys1.s mode normal PSA (a) background curve of the sample, (b) stnppmg curves of the sample, N = 3 For the stnppmg curves (b), the recording ISinterrupted at the begmnmg of the copper stnppmg signal The abctssa scale IS the tune, each mark corresponding to a number of seconds The TMF formation IS made m a HClO, 0325M,Hg++ 0 02 M medium, followmg the procedure described in Rg 2

For blolo@cal samples, __ - lo-15g of the ongmal _. material are generally suniclent to represent a valid samplmg, accordmg to (a) the level of the metal content m the sample, and (b) the method used m the determmatlon[l] In the goal of routme analysis, PSA electrolysts times (equation 1) higher than 1 h are prohlbltlve, the mass of the lyophlhsed material has been adapted to this condition The ongmal material, cut in fine slices, IS accurately weighed m a tared homogemzmg vessel where it was crushed and nuxed with twice its mass eqmvalent of redlstllled water, wlthout salt addition It IS directly submitted to the lyophlhsatlon process overmght The dry powered residual matter IS accurately weighed and an adequate ahquot (0 15-O 30g) IS taken for the subsequent steps, pernutting mdependent replicate measurements (N’) These ahquots, rewetted by 5ml of redistilled water and mlxed for Smm, are treated wrth Sml of solution of the digesting agents and somcated in a reflux apparatus for a predetermined time and temperature At the end of the dtgestlon process, the solution IS transferred to a 50ml volummetrlc flask and rinsed with several portions of no-propanol to a total volume of lOm1 An eventual filtration of the residue can be made on a 45~ cellulose-acetate filtering device and washed Hrlth ao-propanol to obtain a clear solution, but this step has no influence on the PSA measurements Addition of 3ml concentrated lead free hydrochlonc acid and redistilled water to the volumetric flask mark completes the adaptation of the digestion process with the PSA measurements Ahquots of lOm1 of the final solution, permitting the independent N replicate measurements, are

added m the electrochemical cell with a calculated volume of Hg+ + solution of adequate concentration the medium IS directly operational for the VI au thm mercury film (TMF) formation, opt+ mlzatlon of the metal stripping signals and quantitative analysis of the metal content of the animal tissue by the standard addltlon method Dtgestron optrmlzatlon To study the e!Iiclency of the wet digestion process and ensure that the dlssolutlon of the metal ions 1s complete, 0 15-O 30g of a lyophlhsed heifer liver tissue (eventually spiked with metal ion standards) are submltted to low temperature ashmg, a method which 1s churned to completely mmerahse the organic matenal present m such samples[lS, 191 Investigated parameters of the wet digestion are sample mass, DOC and SDS concentrations, somcatlon time, total reactlon time and temperature The reference LTA 1s studied with sample masses from 0 087 to 0 324 g, m definite conditions determined with an oxahc acid standard madent power 6OOW, reflected power 25 W (mm), equlhbnum pressure 2 Hgmm, oxygen flow maxrmum temperature (measured at 50mlmm-‘, the sample level) 13O”C,duration 2 h, followed by a restoration to the ambient temperature and a last 1 h ozomsatlon cycle, N = 8, samples positioned at random m the four rooms of the instrument The ashed white-brown residues are, at the end of the process, directly and totally soluble m the perchlorlc acid “reference” PSA electrolyteC6, 7, 91, a medium free of complexatlon properties both with M(n+) and Hg++ ions and free of adsorption phenomena at the TMFE interface In such condltlons, the perchlonc acid solutions, spiked with a Hg+ + standard, gve directly the strlppmg signals corresponding to the

320

CH LABARand L LAMSEg~

metal tons present In the heifer hver (Ftg 1) These stgnais are finally analysed by a standard addttton method and correspond to a 100% reference efficiency In wet dtgestton processes, the corresponding efficiency hes between 98 3 and 103 2%, with a mean value of 1009% (N = 8) when the mvesugated parameters are sample mass 0300g maximum, DOC and SDS concentration 0 1% m wetght each, 80°C, sonlcatlon t~me 1 h (mm), total reaction time 3 h These conditions wtll be systemattcally used m the followmg paragraph wtth other types of animal tissues When DOC and SDS concentrations are htgher, reaction ttme is shorter, but PSA background curves (as shown in Ftg 1) become so ~mportant that the correcuon ts necessary for quantitative analysis The dtgest~on efficiency ts d~fferent for each metal f l, after 1 h somcaUon at 80°C, only 30% Cd, 45% Pb and 68% Cu are determined m the digesting medmm Note finally that LTA process and wet dtgestton consume globally the same time (approximately 3 h) ~f one conslders that, m each case, the adaptation to PSA is tmmedtately made The main hmltat~on of the LTA ~s the questton of a representaUve samphng and the sample mass whtch can be ashed m the instrument re, wtth 0 5 g aliquot, correspondmg approximately to 9 5 g of the ortgmal material (lower than the level usual m standardtzed methods[l]), the LTA needs 6 - 7 h of treatment, whereas the wet digestion remains accomphshed in 3 h Adaptatwn of the &oestton medmm to PSA Isopropanol, w~th 20% vol composttton m the electrolyte medmm, appears here as a result of the study of the advantages of mtxed (orgamc-aqueous) electrolytes m electrochemical methods[20, 21] Isopropanol mh~btts the competmve reductton of protons durmg the electrodeposmon step (reduction of M(n +) to M(Hg) and Hg + + to Hg(l), see theory) m htgh acadlc condtttons[6, 12] wtthout havmg complexatlon properUes both wRh M(n +) and Hg ++ ~ons On the other hand, the low pH, due to the 6% vol concentrated HCI, keeps the final electrolyte m tts opUmum pH range for PSA of metal ions[6] and the M(n +) ~ons in thetr free (hydrated) form[22] When low metal content Is suspected m a sample (below the level of 10/zgkg -1 of the original

material), tso-propanol IS replaced by glycerol, a substance havmg the same effects but rising drasUcally the dynamtc viscosity of the electrolyte and therefore permitting the measurement under dlffusmn control during the strippmg step ("stationary electrode" PSA[7, 8]) and enhancmg finally t, for the same td Both alcools also are good solvents for DOC and SDS, permitting relauvely high concentraUons of these chemlcals tn the d~gestlon process It is now necessary to carefully study the analytlcal PSA condRtons m thls type of electrolyte Determination of the PSA operatmy condzttons Selectwn of the electrodeposttton voltage, Ed When s~multaneous analysts of the different metal tons of a sample ~s needed, the selectton of E d is experimentally obtamed by a stepwlse variatmn of the electrolysls voltage from the value needed for mercury film formatton ( - 0 40 V/sce) to more n e g a t i v e values The opttmum E d value for the group of metal ions present m the sample ts the value where the strippmg time ratms of selected metal runs parrs (rattos whtch are independent of electrode surface lrreproducttbdttles and of accadental mstrumental fluctuaUons, as in the internal standard methodology) reach a constant value mdependently of the actual electrodeposRlon voltage setting In Ftg 2, stripping ratms of the two metal pairs detected in a heifer hver sample (Cu-Pb and Cu-Cd, respectively) are determmed as functions of the electrolysis voltage E d (V/sce) When E d is more anodlc than - 0 6 0 V , the sole strippmg signal is Cu, at E d = - 0 60 V, ts ~ reaches tts maximum value and the Pb stgnal appears with a stnppmg time only (approxtmately) twice the mmtmum readable value (02s), for E d more anodlc than - 0 8 0 V , the Cd signal is not vtsuahzed on the strtppmg curve, for E d = - 0 8 0 V , tscd reaches tts minimum readable value, whereas t s Pb IS enhanced, finally, for E d more cathodtc than - 1 35V, hydrogen formatmn begins to compete wtth metal deposltmn, leading to stripping time lrreproductlbdRles higher than those obtamed at more posttlve voltages (RDS = 1, 1%, for N = 5) As mdlcated by Fig 2, the optimum electrolyms voltage for the three metals detected m this heifer hver ttssue hes between - 1 05 and - 1 25 V, if only Pb and Cu would be analysed, - 0 80V ts sufficmnt,

5O

~-

0 t Cu/t Pb

40

0

t ts Cu/t s Cd

~ 30 N

20

0 --

0 0

"

10 - -

o

0 ~0

#

0 0 0 ~ 0 0 0 O0 ll$ ~0

O

0 O? 0 0 0

I 8

-16

-14

-12

-10

-08

-06

-0 4

Electrolysis voltage (V vs see) Ftg 2 Selection of the elcctrodepositton potential E d a s a funcuon of the metal ton studtcd t.cu appears as numerator of the stnppmg ttme ratios as it is the first stnppmg signal appeanng when the elcctrolysts voltage becomes more negattve See text for explanations

321

PSA of metals m anunal tissues

II

0 0.2

0.3

I 0.4 Electrolysis

Electrolysis

I 0.5 voltage

1

I 0.6

0.7

(-) E/‘/v (XC)

I

I

0.4

0.5

voltage

(-)

E/V (see)

y =-IO.8 - 29.2xR2=0.983 copper pseudo-polarogram

Electrolysrs

voltage

(-) /Z/V (XC)

In a heifer liver tissue Note the negative sign on the electrolysis voltage scale, m the goal to have the smular figure as a polarographlc wave (a) experunental data (N = 5, RDS Indicated), (b) smoothed data, (c) equation (3) and detemunatlon of the copper stnp ping potential, E, Cu. m the final electrolyte See text for explanations Fig 3 Generation

of a pseudo-stnppmg

polarogram

mercury film formatlon and analysts of Cu alone can be performed with an electrodeposltlon voltage of -06OV When interest IS focussed on a particular metal m a sample, the selection of E, can also be made by the generatlon of the corresponding so-called “stnppmg pseudo-polarograms”[23, experimentally 241, obtamed by measurmg the stnppmg time evolution of the metal of Interest with a stepwlse vanatlon of the electrolysis voltage, from the background value (E,, = 0 lOV/sce) to more negative potentials The optimum electrodeposltlon value corresponds to obtaining the maximum value of t, t,,, , mdependently of the actual electrolysis voltage settmg The

cathodic hmlt of the varlatlon 1s defined as the potential at which the next more reductble metal appears m the strrppmg curve[8] Stripping pseudo-polarograms correspond to a modlficatlon of equation (2) made, as m classical polarography for the chffuslon hnutmg current, m the goal of mcludmg the expertmental data[l l] E, = E” + RTInF In di/D&‘)t,, + RT/nF In(t,,,

- tJt,)

(3)

(d is the diffusion layer thickness, 1 IS the MF thckness) Such plots are used orchnarlly to study the speclatlon of metal ions at concentrations too

CH LABAR and

322

low to permit the direct use of voltammetry, m the same way as analogous plots based on voltammetnc stnppmg curves have been used[25] It 1s mterestmg m thts study to vrsuahse the eventual chelation properties of the drgestron and electrochemtcal media For such purposes, PSA had the advantage over voltammetrtc strtppmg analysts (WA) that the stnppmg polarograms can be generated more qmckly[ 1l] Furthermore, stnppmg polarograms are used to control the efficiency of the forced convection hydrodynanucs at the workmg electrode, to test the reverstbthty of the redox system, to measure the number of transferred electrons and, finally, to obtam correct values of the stnppmg potentials, E,, (potential at whrch the last term of equation (3) 1s zero), rdentrfymg the metal ran stu&ed[26] Figure 3 1s an example of the generation of the copper stnppmg pseudo-polarogram m a heifer liver sample Direct t,, values (background corrected, as demonstrated m Frg 4) as functtons of E,,are presented m Ftg 3a, appropnate smoothmg of these expenmental data[27] are rehandled m Ftg 3b, 40 -

C 2 z

30-

L LAMBWTX

leading by extrapolation to the value of t,,_&, finally, the experimental demonstratron of equation (3) 1s obtamed m Ftg 3(c) From this type of expenment, we obtain n=213 (+/-018), RT/ 2F = 0029V and the correct value of the strtppmg potential of the metal stu&ed (here, for Cu+ + -0035 (+/- 0006)V/see The system appears reversible) Strlpprng potent&s, E, M(Hoj Measurements made by the two previous methods lead to the determrnatton of the actual stnppmg potentrals, E,,(,,, (V/see) of the different metal tons present m the various samples studied These E,M(HIPvalues can be compared to those characterrsmg the stnppmg step in more classical electrolyte media (aqueous solutions of various mmeral actds, with the correspondmg akah salt, at fixed pH values) encountered m PSA[6, 73 Table 1 presents this type of compartson for the three tons present m the actual samples In thts table, all the numerical values of E,Yu,B~ and E,, are reported from[6], m which tt was also demonstrated that pH varrahons between vanous

(4 r,=-008+1 19r, VlrlPncC 1 000 (N’= 3, N = 4) oomsl-PSA. fish bver

ii? ii F 20 t f % 10 u”

Electrodeposttton

I2 10 _ v) z _

s-

ttmelmtn

(b) Cd t, = -0,006 + 0.034 id vrnrnce 1 Pb I, = -0 28 + 0 18 td YP~,P~CC 0 99 stsuonsry electrode-PSA, hclfer hvcr

(N=3,N=4)

20

40 ElectrodeposItIon

60

80

tlmclmm

Fig 4 Relatlonshlp between electrodeposltlon and stnppmg times m normal and stationary electrode PSA m hafer and fish hvers (a) copper analysis by normal PSA, (b) cadmium and lead analysis by stationary electrode PSA t. = 30 s See text for explanations

PSA of metals m ammal tissues

323

Table 1 Stnppmg potentials of Cd, Cu and Pb and background potentials m vanous media The numerical values are gwen wth a absolute mean standard devlahon of 0 OlOV approxunately Medmm

PH

E. cdv/sce)

E. &V/sce)

Mtnc acid Sulfunc acid Perchlonc acid Phosphonc aad Acetic acid Hydrochlonc acid tis study, vanous samples

100 100 100 144 283 100 100

-0615 -0620 -0652 -0640 -0655 -0590 -0 625

-0350 -0432 -0300 -0300 -0 238 -0 342 -0350

medza do not explazn the observed differences between the strrppzng and background potentzal values Concluszozzs can thus be made on the nature of the speczes analysed zn the fizzal electrolytes of the present study (a) EaPb and E,, are qzzalztatzvely zdentzcal zn all electrolytes anions do not have a detemzzed action zn the stnppzng potentzal values, prevzous studzes[4, 5, lo] have demonstrated that, zn such cases, metal zons are zn the “free” (hydrated) fozm zn the electrolyte, and that complexatzon nor adsozptzon process at the TMF interface occur zn solutzon The same concluszon applies for E, the real electron transfer mechanism schematzzed zn the Theory paragraph zs Hg(l)+Hg(II+)+2X--Hg;++2X(b) E,, zs zdentzcal zn HCI, pH 1 medzuzn and zn the acutal final electrolyte thzs fact zs due to the complexatzon propertzes of the chlonde aznon, leading to the conchzszon that, zn such solutions, the oxzdzzed forzn of copper IS, zn fact, CuCl+[5, lo] The same conclzzszon cazz be gzven for the E, Hg(l) + Hg(II +) + 2X - - Hg?X,<,, In the first five medza of Table 1, the first of the preceding chemical equatzon zs prevalent, zn the two last media, the last reaction becomes valzd The fozmatzon of a mercurous chlonde filzn on the TMF must be avoided zn these medza thzs IS automatzcally made, zn normal PSA, by recycling the potential of the working electrode to the electrodeposztzon one when the value of Ebp zs reached at the end of the recorded stnppzng curve (Fzg 4) When low metal content zs expected, “statzonary electrode” PSA[7, 81 must be used, and thzs caution kept zn mind, the automatic recycling of the potentzal being znstrumentally disconnected Note that the adzmsszble potentzal range, ([I$ - E&j), pez-mzts the analysis of other metal zons (such as Zn E, = -1 OSV, Tl E, = -065OV, In E, = -058OV, Bz E, = -006OV) and, eventually, Hg++ (wzth a copper thzn film working electrode and Mn(VII+) as oxzdzzzng agent) zf present zn the animal tissue samples No interference between the metals zs vzsualzzed zn the actual final medium In conclusion, the dzgestzon azzd the adaptation steps lead to medza havzng the same electrocheznzczd properties as a classzcal hydrochlonc acid, pH 10, medium Apart from the known complexatzon of copper and merczznc zons by chlorzde, no evidence appears for other chelation zn solzztzon or interface adsorption effects at the electrode-solutzon Interface The analytical scheme IS thus compatzble wzth PSA

E$V/sce) -0435 -0415 -0463 -0450 -0460 -0420 -0 480

0 023 0 295 0300 0 290 0 280 0090 0100

Relatzonshlp between the electrodeposltzon

tune and

the stripping tzme Equatzon (1) postulates a hnear relatzonshzp between t, and t, Thzs must be confirmed, speczally for long electrodeposztzon tzznes, zn mzxed (aqzzeous-organzc) electrolytes where matrzx and/or organic component of the solzztzon can affect the stnppzng curve by adsorptzon effects at the TME mtezfaceC24, 251 If stationary electrode PSA zs used at long electrodeposztzon times (for low metal content), absorption causes a defozmatzon of the background curve, which zs szmply the chronopotentzometnc image of the double layer capacztance discharge Figure 4 shows vmous examples of venficatzon of equatzon (1) zn both PSA modes In the samples stzzdzed, copper zs systematzcally at higher levels of concentration than lead and cadmium, copper can thus be analysed by normal PSA at short electrodeposztzon tzme [Fzg 5(a)], lead and cadzmum can be analyzed zn thzs mode, but electrodeposztzon times wzll be higher, zn such cases, zt would be preferable to zzse stationary electrode PSA to reduce global analyszs wzth a rest tzme, t, (znterruptzon of the stzrnng of the solution at the end of the electrodeposztzon step, to leave the stzrnng step be done zn a quiescent medzum[7, 9, 111) of 30s (Fzg 4b) Orzgzn ordinate zn stationary electrode PSA shows, especzally for lead-which IS at a more anodzc strzppzng poentzal than cadznzzzzn(Table 1)--a light adsorptzon effect, as was already demonstrated zn such cases[6, 123 thzs zndzcates that the standard addztzon method IS necessary zn routine quantztatzve analyszs of such samples Formatron zn sztu, of the thzn mercury film and reagents blank correctzon uz real samples The TMF at the glassy carbon electrode zs the first step of each PSA measurement Thzs fozmatzon can be achieved zn aqueous aczd solutzons contaznzng an appropriate amount of a soluble mercury salt[B, 83 (Fzg 1) Practically, TMF conservatzon zs not easy to assume zn routine analysis and one must consider that the TMF formation zn the sample solution itself as a prelude to the analytical measurement zs highly preferable Such a process IS valzd only zf the preceding cheznzcal treatments of the sample are compatzble wzth zt The question to solve zs to determine zf the sample (or the residue obtazned before PSA) does not induce supplementary background correction Figure 5 answers thzs questzon by zllustratzng the step-by-step formation of the mercury film on the glassy carbon electrode zn the medzum where PSA tiyszs wzll be effectively done The figure zllustrates, experzmentally, the successzve strzppzng curves

01

324

LABARand L LAMB~RTS

-1

> 4‘

/ illL-ml I 1I CU

0

a

b

e

d

c

f

h

g

k

1

T

Frg 5 FormatIon of the thm mercury lilm on the glassy carbon electrode and reagents background stnppmg curves m the final supportmg electrolyte (a)-(g) TMF formation by four Successiveelectrolysrsstnppmg curves wtth deposition potentials E, rangmg from -0 30 to -0 60 V/see by 0 050 V steps Electrodeposition tune td 1 mm, recorder speed 5 s cm-’ (h)-(l) reagents background stnppmg curves at the last electrodepositlon otenhal for t,, respectively, 2,4, 8, 16 and 32mm, recorder speed 5 scm-’ At this stage, optmuzatlon of the sample signals (Fig 1) can be made The abclssa @me) mark corresponds to 5s The procedure IS valid for lllgh metal content (see text) For metal content, stationary electrode PSA IS recommended at the end of step (l) Electrolyte composition DOC 001% w , SDS 001% w Iso-propanol 20% vol , water 74% vol , [HCI] = 075M, mg++] = 8 24 x 10W4M, h&er hver sample

recorded durmg the TMF formation (according to the procedure developed by Jagner[S] and Jagner et al [S]), signal optlmlzatlon (depending upon the actual metal contents of the particular sample stu&ed) and stnppmg signals obtamed at v~tlous electrodeposltlon times, before standard addition increments This figure shows, re that a light background correction is necessary at the stnppmg potential of copper analysis at electrodeposition time of 32mm, while, indicated by Fig 4a, the Cu content can be quantitatively determined (signal/background ratlo > 10) for t, m the range l-2 mm The same conclusion can be pven for Cd and Pb deterrmnatlon m stationary electrode PSA 15 as gE

: 4. Egg u E ~OSc =

10 -

Selection of the optzmum Hg+ + concentration It 1s quite acadenuc to solve this question m practical analysis, because the optimum oxldlzmg agent concentratlon depends pnmanly on the metal ion content of the sample If mercunc ions concentration is “too” high, the stnppmg reaction 1s so rapid that the strlppmg time cannot be measured Hnth the ISS 820 classical mstrumentatlon used (t,mln = 0 2s at the mammum REA 80 recorder speed), if the same concentration 1s “too” low, lrreprodutiblhtles m the recorded strlppmg times inhibit the quality of the quantitative analysis (wth electrodeposltlon times compatible with routme measurementsC5, 6, lo]) Furthermore, as indicated m the preceding para-

Cd rs= 1 15 + 0 02 q M(n+) vrnrnce 0 99 Pb ts= 0 60 + 0 02 q M(n+) vrrIance 0 98 Cu I~= 5 51 + 0 06 q M(n+) v~rnnce 098

./

0 * c

a$’ %o”$ :=a G%z

s-

E 2. 2 38 -0200

-100

0

100

200

Quantity M(n+) added&g Fig. 6 Standard addition method applied to a fish hver sample of the Mosan river Copper IS analyzed by normal PSA, cadmium and lead by stationary electrode PSA, m two ahquots of the same final solutton, increment ad&Ions depend on the tmtial concentration m the sample Operational condlttons Identical to Ftg 5

PSA of metals m animal tissues graph, mercunc ion concentration must be sufficient to ensure good TMF formation The question of optimum mercuric concentration 1s thus an mstrumental (recording detection hmlt) and technical (cell configuration, use of mlcroelectrode for radial dlffuslon or macroelectrode Hnth a glassy carbon spray) problem For the various samples analysed m this study, a mercunc ion concentration m the range 10m4lo-’ M m the electrochemical cell reahzes the above condltlons Prehmmary measurements must be done by varying, step by step, this concentration by a factor 10 when no qualitative mdlcatlon 1s gven for the metal content of a partrcular sample Analyt~al applrcatzons Figure 6 1s an example of the standard addltlon method applied to a fish liver sample m operatlonal PSA condltlons defined m Fig 5 In general, as the copper content 1s higher than lead and cadmium contents, the first of the metal ions can be determined by normal PSA (electrodeposltlon times t, m the range of 4-8mm) and the last two, by stationary electrode PSA (electrodeposltlon times t, m the range 15mm, rest time t, of 30s) Additions of 4-8 pg ahquots of Cd and Pb are generally adapted for the various samples examined, whereas, for Cu analysis, ahquots of 10-25pg are preferred RSD are m the order of + 5% (Pb, Cd) and f 3% (Cu) for N = 4-7 CONCLUSION Tremendous tentatlves have been done to arcumvent the organic matter total destruction as the mltlal step of analysis of blologcal matenals Electrochemlcal methods based on a current measurement often have some problems due to this step (background correct evaluation, mterference of peak currents and shifting of the electrochemical charactenstlcs) A softer process can be directly adapted to potentlometrlc stnppmg methods and can be used m the field of routme measurements, Hnthout costly mstrumentatlon nor high levels of technical knowledge The analysis of lyophlhzed animal tissue mater&s requires approximately 3 to 4 h of operation when applymg the standard addltlon procedure The results demonstrate the inherent advantages of a no-current electrochemical technique Acknowledgements-Our Neurolo@c Department,

thanks are due to M Dumortler, Chmques Umversltalres Erasme,

325

ULB, Belgum, for Ius snentdic assntance m LTA measurements and to J C Mocha, Umtk d’Ecolo@e des Eaux Deuces, Facultes Umversltmres Notre-Dame-de-la-Paix, Namur, Belgmm, for sampling, conservation and methods of use of the fish hvers

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